Ireneaus's Against Heresies was published in about 180 CE. The original Greek title, On the Detection and Overthrow of So-Called Gnosis, is called Adversus Haereses in Latin. The plurality of the title denotes that Irenaeus will address himself to several sects of a major type of heresy in circulation during his time: Gnosticism.

Against Heresies is a product of the post-Apostolic age (i.e., the period immediately following the generation of Christ's apostles). Irenaeus was a pupil of Polycarp (both are from Smyrna, in modern Turkey), who himself was a pupil of the gospel writer John. That is to say, Irenaeus himself did not know the apostles but looked to canonize the gospels that were in circulation. He was sent to modern Lyon, France, in 161 to serve as bishop.

Against Heresies consists of five books. The first is a description of heretical sects, the second is a refutation of those sects, and the third, fourth, and fifth books constitute an exposition of the true Christian doctrines, including the incarnation of Jesus, the nature of God, and the resurrection. The primary target of Irenaeus's work was one Valentinus, an Egyptian-born theologian and proponent of Gnosticism. Gnosticism claimed that Jesus passed along secret knowledge, known only to the apostles by means of oral tradition. Thus, Gnosticism is a form of mystical religion, in which only a few select initiates are instructed. Valentinus gave rise to Valentinianism, which, in addition to promoting the basic views of Gnosticism, includes a complex cosmology of a creator (demiurge) who is fundamentally different from the monotheistic God. Valentinus's demiurge is imperfect, and the material world (according to his and other strains of Gnosticism) is evil.

In addition to being a primary source of modern knowledge of ancient Gnosticism, Against Heresies is also a great source for understanding the ancient process of canonization. Irenaeus quotes from nearly all books of the Bible and clarifies that the God of the Old and New Testaments is the same.

Finally, Irenaeus was a proponent of the idea of apostolic succession of bishops. That is to say that the apostles themselves chose their successors, and this selection process continued in an unbroken line of anointed bishops.